Head towards the plaza area, fighting or evading Turtle and Driftwood Pirates along the way. Once the plaza is reached, a short cut scene will launch, then it's time for a final battle against Maccus. Maccus is the only enemy that actually has to be fought to complete this mission. Pirate Bombs will do the trick nicely and from a distance. Hit him enough times and the mission will complete, revealing the final piece of the Kraken's Bane.